Presently, laboratories working with cell cultures use plastic disposable vessels; such vessels have different shapes such as cylindrical bottles, flat round dishes or polygonal (e.g., triangular) flasks. Polygonal flasks are the most widely used in tissue culture for a number of reasons. For example, the surface area may be changed by using them in the upright or the flat position. Conventionally, this type of vessel has a closure or cap that can be closed air tight once the flask is in a non-sterile environment and can be left partially open when the flask is in a sterile incubator or the like to allow gas circulation in the flask (necessary to maintain the appropriate pH).
During the manipulation of the flask (seeding of cells, change of tissue culture medium etc.) the surface of potentially contaminated or contaminable area is smaller that in some other type of vessels such as tissue culture dishes wherein the lid has to be completely removed to allow manipulation of cells and thus the whole surface of a cell culture is exposed to contamination by external agents.
Flasks that are commercially available at the present time are all characterized by a screw cap positioned on the flask's neck which is on the top of a containment chamber. The use of a screw cap as a flask closure presents several practical problems, as follows:
1. Sterility
Most of the manipulations of cell culture flasks are performed under a sterile hood where a vertical laminar flow of sterile air blows down from the top of the cabinet onto the work surface and is drawn through the work surface and either recirculated or vented. Any object that interrupts the air flow cause a turbulence in the area below the object; turbulence draws in surrounding air which is not sterile. Furthermore, objects and instruments (including the hands of the operator) used under the hood, are sometimes not sterile; thus they can release small particles of powder or other materials that contain contaminating particles.
2. Training
The operators usually handle the samples to be injected in or withdrawn out of cell culture flasks using electrical instruments called pipettors; such instruments allow controlled dispensing of liquids through disposable pipettes. This means that an operator has one hand fully occupied working with the pipettor leaving the other hand for all the operations necessary to unscrew, open, displace, replace, close and screw the cap of the flask. These operations need to be done in such a way that no contaminated surfaces (external part of the cap, fingers, etc.) contact the top of the open flask and require a lot of operator training to be properly performed.
3. Contamination of the Sample/Operator
Still, a well trained operator can either contaminate the flask neck or be contaminated by the microorganisms present on the flask neck if the flask or the closure must be handled in a non-sterile area away from the sterile atmosphere below the hood, or if the flask closure is placed in the non-sterile outside area while the flask is manipulated. Sometimes this becomes necessary because of the difficulty in handling the flask and screw type closure.
4. Working Speed
Operations such as injecting and withdrawing of samples, and removing and replacing closures, can be properly done with a screw-type closure only by using both hands; this is not always possible and when it is possible, it is inevitably slow.
5. Contamination of the Working Area
Furthermore, if two hands are used the cap has to be placed on the surface of a bench outside the sterile area while operating with the flask; this exposes the cap either to be contaminated by external agents or to contaminate the bench or other objects in the working area.
One example of a cell culture flask with a screw cap which is also hinged to the flask so as to be movable by the finger of one hand to an open position after the cap has been unscrewed, while the flask is held by the same hand, is disclosed in Kassis et al. U.S. Pat. No. 4,534,483. To provide a reuseable cap this patent discloses a screwcap connected to the flask by an orientation strip which is attached to a collar which encircles the neck of the flask and freely turns about the neck to allow the cap to be threaded onto threads outside the neck of the flask. To open such a cap from the closed position requires two hands, one to hold the flask and one to unscrew the cap. As above noted, this requirement to use two hands to open the cap on the flask raises numerous practical problems.